Caffeine Confuses Your Body's Internal Clock, Study Suggests

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Drinking a cup of coffee at night may mess up your sleep in more
ways than one: Caffeine not only keeps you awake but also affects
your body's internal clock, which tells you when it's time to
sleep and wake up, a new study suggests.

The finding shows that caffeine "affects our physiology in a way
that we hadn't really considered in the past," said Kenneth P.
Wright Jr., a co-author of the study and director of the Sleep
and Chronobiology Laboratory at the University of Colorado.

The research may have implications for
night owls who want to get up earlier, and for people who
experience jet lag after traveling between time zones, the
researchers said.

In the study, five healthy people spent a night in a laboratory,
where researchers monitored their levels of
melatonin, a hormone that increases at night and is an
indicator of the body's internal clock, also known as circadian
rhythm.

Three hours before bedtime, participants took either a pill that
contained about as much caffeine as two shots of espresso, or a
placebo. (The experiment was repeated on another night so that
participants who received the caffeine instead received the
placebo the next time, and vice versa.)

The caffeine pill delayed the body's release of melatonin by
about 40 minutes, the researchers found. In other words, caffeine
affected the body's
internal clock, so that the start of "biological night" was
pushed later, the researchers wrote in their findings.

Researchers have long known that caffeine causes the release of
chemicals in the brain that promote wakefulness and arousal,
Wright said. But the new findings show that caffeine also affects
the body in another way, by influencing circadian rhythms.
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"We think this is another reason why we may have some sleep
disruption" when we have caffeine before bedtime, Wright told
Live Science.

A 40-minute delay in the release of melatonin could have a
relatively large effect on a person, Wright said. For comparison,
when people are exposed to bright
light before bedtime — which is known to have a strong
influence on the circadian clock — their release of melatonin is
delayed by about 85 minutes.

Previous studies have suggested that people who consider
themselves "night owls" tend to consume more caffeine than those
who are early risers. The new findings suggest that consuming
caffeine in the evening may play a role in why some people are
night owls, Wright said.

If a night owl wants to start getting up earlier, "removing
caffeine in the evening and afternoon" may be one way to help,
Wright said.

In addition, a dose of caffeine that's properly timed may be able
to help people better adjust to jet lag when they travel from
east to west (and thus need to push back their internal clock),
Wright said. However, more studies are needed to examine this
idea, because caffeine might also make jet lag worse, Wright
said.

The study is published today (Sept. 16) in the journal Science
Translational Medicine.